Sandi....I will have to assume some things here because of the lack of info.
A clunking sound can be caused by several things. I list the most common below.
1. Transmission shift operation fault.
2. loose engine/transmission/transfer case mounts.
4.loose spring mounts.
5.loose pinion gear nut and yoke.
6. excessive ring gear backlash.
Your mechanic should be able to duplicate and isolate this clunking to diagnose your problem. If he can't.....I would seek out a better trained mechanic. If this is a KIA dealer......I would take my complaints up the line and be sure to document all your service calls.